The present invention relates to polyamic acids ("PAA"), to a method for converting PAA into polyimide-benzoxazole ("PIBO") polymers, and to films and coatings of PIBO.
In the preparation of polyimidebenzoxazole, a dianhydride and diaminobenzoxazole are first reacted to form a polyamic acid. The polyamic acid is then converted to PIBO by closing the amic acid linkages to form imide rings. Heretofore, only a limited number of dianhydride/diaminobenzoxazole combinations have been disclosed for producing PIBO. While these limited number of PIBO polymers have been formed into fibers and films, the resulting fibers and films do not generally have sufficient physical and/or chemical properties (such as tensile modulus, tensile strength, elongation-at-break and coefficient of thermal expansion (CTE)) to make them generally useful in applications such as for the electronics industry.
For example, U.S. Pat. No. 4,087,409 to Preston "Ordered Heterocyclic Copolymers" broadly teaches reacting two symmetrical monomers to produce an essentially linear heterocyclic polymer having at least two different heterocyclic linkages. Polyamic acids can be prepared by selecting monomers from an otherwise long list of monomers (e.g. 2,2'-p-phenylene bis(5-aminobenzoxazole) and pyromellitic dianhydride). Using the exemplified techniques, non-benzoxazole-containing polyamic acids are prepared at inherent viscosities of 1.13 dL/gram. Films made from these polymers and techniques are described in summary fashion, without specific processing techniques for preparing the films or physical properties disclosed.
"Fibers from Ordered Benzheterocycle-Imide Copolymers," Appl. Poly. Sym., No. 9, pp. 145-158 (1969) describes the preparation of amic acid polymers by polycondensing specific aromatic dianhydrides and aromatic diaminobenzoxazoles. The polyamic acids exhibit inherent viscosities of 1.5 dL/gram and less. Without disclosing, in detail, the specific techniques, the polyamic acids are formed into PIBO fibers. These fibers have less than desirable physical properties, particularly tensile strengths.
"New High-Temperature Polymers, VIII Ordered Benzoxazole- and Benzothiazole-Imide Copolymers," J. Poly. Sci., Part A-1 (1969), 7(1), pp. 283-296, describes the preparation of ordered heterocycle copolymers prepared from aromatic dianhydrides reacted with aromatic diamines containing preformed benzoxazole or benzothiazole units. The ordered heterocyclic copolymers were prepared in their soluble polyamic acid form with inherent viscosities no greater than 1.78 dL/gram and fabricated to the desired film, fiber or metal coating. After fabrication, the polyamic acid is converted to PIBO such as by forming a film from the polyamic acid solution, and heating the film for 20 minutes at 100.degree. C., 20 hours at 145.degree. C. and then 300.degree. C. for 1 hour. Films made from these polymers are described in summary fashion, using terms like "brittle" or "strong."
"Azole Analogs of Polypyromellitimides," Vysokomol. Soed., Vol. (A) XIII, No. 11, 1971, pp. 2565-2570, discloses synthesis of azole-containing analogs of poly-N,N'-(p,p'-phenoxyphenylene)pyro-mellitimide!. Structure V in this paper is a PIBO prepared from 2,6-di(p,p'-aminophenoxyphenyl)benzo-1,2-d:5,4-d'!bisoxazole and pyromellitic dianhydride. The inherent viscosity of the particular polyamic acid used to make this PIBO is 2.5 dL/gram.
Similarly, "Correlations Between the Properties and Structures of some Polyhetero-arylenes," Vysokomol. Soed., Vol. (A) XIV, No. 10, 1972, pp. 2174-2182, in comparing the mechanical properties of other polyimides with those of polyheteroarylenes of similar structure, shows a PIBO polymer in Structure II. The inherent viscosity of the particular polyamic acid used to make the described PIBO is 2.5 dL/gram or less. The physical/chemical properties of the film made from these polymers range from poor to moderate.
U.S. Pat. No. 4,866,873 to Mukai et al. also discloses an aromatic heterocyclic polyimide comprising substantially equimolar amounts of a specific aromatic, trans-benzobisoxazole or trans-benzobisthiazole diamine such as 2,6-(4,4'-diaminodiphenyl)benzo1,2-d:4,5-d'!bisoxazole and a specific aromatic tetracarboxylic dianhydride such as pyromellitic dianhydride. The polyamic acids for these polyimides are prepared in an amide solvent and converted to PIBO. Inherent viscosities of suitable polyamic acids are described as being from 0.5 to 20 dL/g. Japanese Patent Application No. 2-41819 to Mitsubishi Kasei Corporation describes similar PIBO polymers from a polyamic acid made from specific aromatic tetracarboxylic dianhydrides such as pyromellitic dianhydride and specific aromatic, trans-benzobisoxazole or trans-benzobisthiazole diamines such as 2,6-(4,4'-diaminodiphenyl)benzo1,2-d:4,5-d'!bisoxazole. The polyamic acids have an inherent viscosity of at least 0.1 and preferably 1 or more dL/gram. Similar polyamic acids having an inherent viscosity ranging from 2.19 dL/gram to 3.07 dL/gram and PIBO polymers made therefrom are also described in "Novel Aromatic Heterocyclic Polyimide (PIBT) Having Ultra-High Modulus of Elasticity" by Seiichi Nozawa, Kagaku to Kogyo, 44(7), 1154 (1991).
"Synthesis and Mechanical Properties of Novel Polyimide Containing Heterocycles" by Nozawa, Taytama, Kimura and Mukai presented at the 22nd International SAMPE Technical Conference (held Nov. 6-8, 1990), teaches synthesis of one type of trans-PIBT and trans-PIBO using a polyamic acid of 4,4'-diamino phenylenebenzobisthiazole and pyromellitic dianhydride with thermal and chemical ring closure techniques. The polyamic acid used to prepare the PIBO had an inherent viscosity of 3.09 to 3.79 dL/gram. The polymers prepared from the specific aromatic, trans-benzobisoxazole or trans-benzobisthiazole diamine and pyromellitic dianhydride are prepared as fibers.
European Patent Publication No. 0355927 to Asahi discloses a polyimide precursor comprising a specific polyamic acid ester, polyamic amide or polyamic acid salt structure derived from a tetracarboxylic acid compound and a specific diamine compound exhibit low solution viscosity. The monomers used to make the polyimide precursors are generally rigid and symmetrical and each polymer repeat unit contains a pendant organic group having 1 to 20 carbon atoms. The polyamic acid precursors have reduced viscosities from 0.1 to 2.0 dL/gram. The polymers form a photopolymerizable composition with a photopolymerization initiator. The resulting PIBO polymer can be used to coat substrates for subsequent use in photolithography. Films produced from these cross-linked polymers exhibit relatively low tensile strengths and tensile modulus.
Japanese Patent Application No. 41-42458 (Patent KOHO No. 45-8435) to Taoka Senryo Seizo K.K. teaches the synthesis of PIBO from a polyamic acid with the polyamic acid being prepared from an aromatic diamine containing a single benzoxazole ring such as 5-amino-2-(4-aminophenyl)benzoxazole and an aromatic tetracarboxylic acid dianhydride using thermal ring closure techniques.
In view of these deficiencies, it remains desirable to prepare PIBO polymers which have a more desired combination of chemical and physical properties than those previously reported. In addition, it remains desirable to convert these PIBO polymers into useful films and coatings.